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Date:   Sat, 19 May 2018 14:32:55 +0200 (CEST)
From:   Thomas Gleixner <tglx@linutronix.de>
To:     Dou Liyang <douly.fnst@cn.fujitsu.com>
cc:     x86@kernel.org, linux-kernel@vger.kernel.org, mingo@redhat.com,
        hpa@zytor.com
Subject: Re: [PATCH] x86/idt: Simplify the idt_setup_apic_and_irq_gates()
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On Thu, 26 Apr 2018, Dou Liyang wrote:

> The vectors between FIRST_SYSTEM_VECTOR and NR_VECTORS are special IRQ
> vectors used by the SMP architecture. But, if X86_LOCAL_APIC=n, it will
> not be used, and the FIRST_SYSTEM_VECTOR is equal to NR_VECTORS.

Correct, but that function has nothing to do with FIRST_SYSTEM_VECTOR.

> diff --git a/arch/x86/kernel/idt.c b/arch/x86/kernel/idt.c
> index 2c3a1b4294eb..8b4174890706 100644
> --- a/arch/x86/kernel/idt.c
> +++ b/arch/x86/kernel/idt.c
> @@ -317,15 +317,16 @@ void __init idt_setup_apic_and_irq_gates(void)
>  		set_intr_gate(i, entry);
>  	}
>  
> -	for_each_clear_bit_from(i, system_vectors, NR_VECTORS) {
> +	/*
> +	 * If X86_LOCAL_APIC=n, the FIRST_SYSTEM_VECTOR is equal to NR_VECTORS
> +	 * Just consider the X86_LOCAL_APIC=y case
> +	 */
>  #ifdef CONFIG_X86_LOCAL_APIC
> +	for_each_clear_bit_from(i, system_vectors, NR_VECTORS) {
>  		set_bit(i, system_vectors);
>  		set_intr_gate(i, spurious_interrupt);
> -#else
> -		entry = irq_entries_start + 8 * (i - FIRST_EXTERNAL_VECTOR);
> -		set_intr_gate(i, entry);
> -#endif
>  	}
> +#endif

That change breaks the LOCAL_APIC=n case in a subtle way. What the function
does is to set _ALL_ entries starting from FIRST_EXTERNAL_VECTOR up to
NR_VECTORS in the IDT to a known target, except those which are already
occupied by a system vector.

So for APIC=y this sets them to: spurious vector and for APIC=n it sets it
to the corresppnding vector entry. You remove the latter...

Thanks,

	tglx